1. Field of the Invention
The present invention relates to a graphics data handling system for a computer aided design (CAD) workstation. The system maximizes rapid accessibility to arbitrarily selectable two dimensional views of a complex site stored in three dimensional representation by a host computer.
2. Description of the Prior Art
In many computer-aided design applications, a host computer establishes and stores a three dimensional representation of the item being designed. For example, in the design of a chemical process plant, the host computer may assemble a site representation of the plant which consists of a list of the thousands of pipes, valves, fittings and equipment interconnections that comprise the plant. The site list may include a geometric description of each such component, together with the three dimensional coordinates which spatially locate each component within the plant site.
Advantageously, each engineer working on the plant design will have a workstation or graphics display system that interfaces with the host computer and facilitates the display of selected images of the plant being designed. Such a workstation would permit the engineer to rapidly display two dimensional views, from arbitrary viewpoints, of arbitarily selectable portions of the plant represented by the site lists in the host computer. One objective of the present invention is to provide a graphics data handling system to facilitate such rapid selectable viewpoint image display in a CAD workstation.
A particularly useful workstation configuration is disclosed in the inventor's copending U.S. patent application entitled "Graphics Display System With Viewports Of Arbitary Location And Content", which was filed on Nov. 2, 1982 as Ser. No. 438,476. That patent application, which is assigned to Cadtrak Corporation, the assignee of the present application, is incorporated herein by reference.
In a workstation incorporating the graphics display system of patent application No. 438,476 individual viewports or video images of arbitary arrangement, number, size and content may be produced on the video screen. Thus e.g., the designer may arbitrarily select which views of the plant are to be displayed. For example, he could simultaneously display a plan or elevation view of a major portion of the plant, together with an enlarged perspective view of the immediate portion of the plant piping which is undergoing design. He can use a panning capability to move the displayed viewport image across nearby or distant portions of the plant, and can zoom in to obtain enlarged views of plant details.
There are certain constraints that limit the display flexibility of such a system when utilized with a host computer that maintains what may be a massive site list. For example, if a two dimensional plan or elevation view of a certain portion of the plant is desired, relatively time consuming algorithms must be used to convert the three dimensional site list information into a two dimensional representation that is capable of being displayed on a video screen. The processing includes culling from the site list the descriptors of all of the plant components which would appear in the desired two dimensional view, and converting the geometrical descriptors of these components and their three dimensional spatial location information (as contained in the site list) into appropriate two dimensional vector or raster representations. Additional processing is required to remove "hidden lines and surfaces" from the resultant two dimensional transformation (i.e., to remove from the final display the portions of components which, though present in the plant at the displayed area, would be hidden from view by other components present in the two dimensional display). Substantial computer processing time, typically hours, is required to perform such 3D-to-2D conversion and hidden line removal. If the designer should then wish to view a different portion of the plant, even one relatively close to, but not contained within, the previously transformed region, another time consuming transformation and hidden line removal computation must be carried out. Minutes or hours may pass before the new image is available to the designer.
Another object of the present invention is to provide a graphics data handling system which maximizes the amount of two dimensional graphics data available to the designer, without requiring additional time consuming 3D-to-2D transformations to be performed. To this end, another objective is to provide a system in which backdrop viewfiles are established in advance, which comprise two dimensional vector representations of substantial subportions of the site represented in the host computer. The video images are derived from these backdrop viewfiles, which are maintained at the workstation. The designer can rapidly access the design information from these backdrop viewfiles for display with panning and zoom. By precomputing backdrop viewfiles for each of the plant subsystems on which the designer is working, the designer will have instantly available to him views of all of the local and surrounding areas of the plant which he is likely to use during a design session. There is no computation delay involved with panning within this region.
A further object of the present invention is to provide a graphics data handling organization that readily facilitates the generation and storage of backdrop viewfiles and the arbitary selection and display of viewport images of portions of the data contained within such viewfiles, with arbitary panning and zoom capability. To this end, it is an object of the present invention to provide an organization in which graphics data is handled in a set of different coordinate systems that (a) maximize the arbitary availability of such data for video display, while (b) minimizing the requirements of data storage and complex computation of image transformation.